Photosensitive printing elements typically comprise (a) a support layer; (b) one or more layers of photopolymerizable or photocurable material; and (c) optionally, a removable coversheet that protects the printing element during transport and handling. Processes for the creation of a relief image in the printing element are well known in the art. These processes can be divided into two general categories: (1) “conventional” methods in which a negative is contacted with a photocurable layer and the photocurable layer is exposed to actinic radiation through the negative; and (2) “digital” methods in which an in situ negative is formed directly on the photosensitive printing element. Digital methods have been increasingly studied as a means to go more quickly from plate to press.
Digital methods create a mask image in situ on or disposed above a photocurable layer of a photosensitive printing element. Generally, any thermally removable layer that is capable of blocking actinic radiation can be used to form the in situ mask image. The in situ mask image remains on the printing element for subsequent steps of imagewise exposure to actinic radiation and thermal development to create the relief image printing element.
Digital methods of creating the in situ mask image typically require one or more steps to prepare the photosensitive element prior to imagewise exposure. Generally, digital methods of in situ mask formation either selectively remove or transfer the radiation opaque layer, from or to a surface of the photosensitive element opposite the support. Preferably, the thermally removable layer is sensitive to infrared radiation in order to carry out the digital methods for forming the mask image with infrared laser radiation. Infrared laser exposure can be carried out using various types of infrared lasers, which emit in the range 750 to 20,000 nm, such as diode lasers emitting in the 780 to 2,000 nm range and Nd:YAG lasers emitting at 1064 nm.
Various methods currently exist for creating digital masks in situ. For example, the photosensitive element may initially include the thermally removable layer as a radiation opaque layer that covers or substantially covers the entire surface of the photopolymerizable layer. The radiation opaque layer is imagewise exposed to infrared laser radiation to selectively remove the radiation opaque layer and form the image on the photopolymerizable layer, i.e., the in situ mask. This process is described, for example, in U.S. Pat. Nos. 5,262,275, 5,719,009, and 6,238,837 to Fan and in U.S. Pat. No. 5,506,086 to Van Zoeren, the subject matter of each of which is herein incorporated by reference in its entirety.
In another method, the photosensitive element does not initially include the thermally removable layer. A separate element bearing the radiation opaque layer forms an assemblage with the photosensitive element such that the radiation opaque layer is adjacent to the surface of the photosensitive element opposite the support. (If present, a coversheet associated with the photopolymerizable layer is removed prior to forming the assemblage). The assemblage is exposed imagewise with infrared laser radiation to selectively transfer the radiation opaque layer and form the image on (or disposed above) the photopolymerizable layer. Examples of this process are described in U.S. Pat. No. 6,773,859 to Fan et al., in U.S. Pat. Nos. 5,766,819 and 5,840,463 to Blanchett, and in EP 0 891 877 A, the subject matter of each of which is herein incorporated by reference in its entirety.
Alternatively, the mask image may be created on a separate carrier and then transferred by application of heat and/or pressure to the surface of the photopolymerizable layer opposite the support. In this instance, the photopolymerizable layer is typically tacky and retains the transferred image. The separate carrier is removed from the element prior to imagewise exposure. The separate carrier may have a radiation opaque layer that is imagewise exposed to laser radiation to selectively remove the radiation opaque material and form the image. An example of this type of carrier is LaserMask® imaging film, available from Rexam, Inc. The image of radiation opaque material may also be transferred to the separate carrier from another element that has the radiation opaque material by means of laser radiation.
Finally, digital mask formation can be accomplished by imagewise application of the radiation opaque material by inkjet printing. Imagewise application of an ink-jet ink can be directly on the photopolymerizable layer or on another thermally removable layer (e.g., transparent layer) disposed above the photopolymerizable layer of the photosensitive element. In most cases, the maximum image resolution available through direct ink-jet masking of a photopolymerizable layer is lower to that available through other mask formation methods.
Although various methods exist for creating digital masks on photosensitive substrates, a need exists for improved methods that can more quickly and efficiently create a digital mask of the desired image resolution to further streamline the process of creating the relief image printing element.
The inventors of the present invention have surprisingly discovered that a negative donor image, which is the byproduct of a thermal proofer, may be used to create an in situ mask that can be laminated to the surface of a photosensitive printing element and processed in the normal manner to create the relief image printing element.